(Received 28 November 2013;accepted 8 December 2013;online 21 December 2013)

The title compound, C18H23N2O2+·Cl−, crystallizes with two independent cations and anions per cell. Each cation has twofold rotational disorder about the linking vinyl groups but with unequal occupancies [0.963 (5):0.037 (5) and 0.860 (8):0.140 (8)]. The two independent cations are close to being related by an inversion centre but the data does not support the expected centrosymmetric space-group assignment. The conclusion is that the differing rotational disorder has lead to an overall non-centrosymmetric lattice. In the crystal, the mol­ecules pack in layers parallel to (133) and (-13-3), chain-linked with motif C12(7) by the di­hydroxy­propyl O–H⋯Cl⋯H–O hydrogen bonds. Other lattice binding is provided by O—H⋯Cl, C—H⋯Cl and C—H⋯N inter­actions.

Organic push–pull chromophores with large second-order nonlinear optical (NLO) properties are in demand due to their potential applications in photonic devices and optical information processing (Kay et al., 2004; Bass et al., 2001; Prasad et al., 1988). A significant number of organic compounds available in the literature with large molecular NLO responses contain N,N-disubstituted anilines as this nucleus is an excellent electron donor. Due to the lack of tethering functionality of these chromophores, the possibility of synthesizing polymer containing chromophores is restricted. In this work, we have synthesized a new NLO chromophore containing an N,N-dimethyl aniline donor and an acceptor based on the dihydroxypropyl pyridinium chloride. The dihydroxypropyl substituent on the acceptor pyridinium nucleus will allow for covalent attachment of the molecule to a polymer backbone.

The asymmetric unit of the title compound (I) contains two independent 1-(2,3-dihydroxy-propyl)-4-[2-(4-dimethylamino-phenyl)-vinyl]-pyridinium cations (with primed and unprimed labels) and chloride anions almost related by an inversion centre (Fig. 1) in space groupP21. The screw axis and the c glide defining data number, average intensities and ratio of intensity/standard deviations were 61, 1/5, 0.4 and 906, 1.9, 3.1 respectively. The corresponding centosymmetric, and more usual, space group found for these compounds, P21/c was rejected through the small but significant presence of the required glide plane absences; this was confirmed in attempted least squares refinements. At the conclusion of both space group refinements, the difference Fourier maps show the presence of two partially occupied rotational conformers about the alkene atoms (C9═C10, C9'═ C10') apparently confined to the vinyl linking atoms, also related by the same inversion centre (Fig. 2). Inclusion of the carbon atoms located on the Fourier difference map (see experimental) improved the agreement factors by about ~0.7% and results in a featureless difference map. Such rotational disorder is not uncommon amongst compounds containing C═C and C═N linkages: for example see Moreno-Fuquen et al. (2009).

The two cations (Fig 1) are very similar with RMS fits of 0.020 Å and 1.28° (PLATON, Spek, 2009); excluding the dihydroxy end groups, they are approximately planar (maximum deviations from the 18 atom planes are 0.055 (4) & 0.057 (5) Å for atoms C16 & C16') corresponding to the angle between the phenyl and pyridinium rings of 1.6 (2) & 2.8 (2)° for the unprimed and primed cations respectively.

Indications from the final solution were that the structure could be refined in the centrosymmetric space group P21/c (viz. 95% in agreeement according to a PLATON analysis (Spek, 2009), with each molecule except atoms O1 & O1' related by inversion symmetry). The dataset does not support this with 984 glide plane systematic absences found to be weakly but significantly present out of the total set of 38305. Our experience with these planar NLO molecules is that they frequently form crystals with centrosymmetrically related molecules. In addition at the R1 value of 0.049, residual peaks in the same plane as the target molecules formed recognizable partial occupancy two fold rotational cation atoms, about the C9═C10 & C9'═C10' linkages (Figure 2).

The defined atoms were paired with the corresponding two major conformer cation atoms (a & b labels) and corresponding H atoms added in calculated positions where observed on the difference maps. H atoms on the located phenyl pyridinium C atoms were not resolved and so were fixed in calculated positions. All non-hydrogen atoms in the partially occupied minor rotamers were given a single group isotropic thermal parameter, which refined to 0.005 (3) A2. It was possible to model a chemically reasonable model using the SHELXL SAME controlling parameters (but only) with the pyridinium and phenyl rings treated independently & occupancies fixed. We elected to remain with the linked group occupancy refinement model presented here. Using SADI, the bond lengths C9═C10, C4–C9, C10–C11,C11—C12 and O–H were restrained to the same lengths. The final occupancies for major(A): minor(B) rotamers were 0.963 (5):0.037 (5)(unprimed a:b) and 0.860 (8):0.140 (8) (primed a:b).

Six reflections with Fo<<Fc at low angle were omitted on the basis of background scatter and 2 were OMITted as outliers with Δ|(Fo2-Fc2)|/σ(Fo2) > 4.5. A l l carbon-bound H atoms were constrained to their expected geometries [C—H 0.95,0.98, 0.99 Å] and refined with Uiso 1.2 times the Ueq of their parent atom except for the minor rotamer(b) H atoms with Uiso=1.5Ueq of their parent atom. All other non-hydrogen atoms were refined with anisotropic thermal parameters.

Fig. 1. Labelling of the major(a) conformer cations and anions of the title compound, with 50% thermal ellipsoids (Farrugia, 2012).

Fig. 2. The total contents of the asymmetric unit, as for Fig. 1, with the minor cation rotamer (labelled b atoms) shown with filled bonds and major rotamer atoms (labelled a) connected with dotted bonds. The centre of the rotation is shown by inclusion of the C9 & C10 (a & b)labels.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Acknowledgements

This work was supported by the New Zealand Foundation for Science and Innovation grant (contract C08X0704). We thank Dr C. Fitchett of the University of Canterbury, New Zealand, for the data collection.

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